1. To make the onion sauce, sauté the onions by adding oil, salt and pepper and blend them.
2. In a pan add 2 tbsp vegetable oil to grease it all over.
3. Then add the onion sauce into the hot pan and stir fry for about 2 minutes.
4. Then add the ginger paste and grated garlic and stir for a second or 2.
5. Add yogurt mixing immediately.
6. Add the cumin coriander powder and paprika powder (if desired) and mix well cooking for 2 minutes.
7. Add the cumin seeds, fenugreek and mustard seeds at a time and mix well.
8. Add the jaggery letting it to melt for a while in the pan.
9. After 2 minutes, add tamarind puree and a tablespoon of water.
10. Season with some salt as per taste and add half a cup of water.
11. Add the cooked prawns and stir.
12. Cover with a lid and let it to cook for about 7 minutes.
13. Check for salt and seasonings.
14. Take a serving plate, spread some cooked spaghetti and serve the prawns over it.